Electromagnetic flowmeters, as is well known, measure the volumetric flow rate of an electrically conductive liquid flowing in a measuring tube. The part of the measuring tube which comes into contact with the liquid is electrically nonconductive so that a voltage induced by a magnetic field according to Faraday's law of induction would not be short-circuited if the measuring tube were completely made of metal, i.e., if the liquid came into contact with the metal. The magnetic field generally cuts the measuring tube perpendicular to the longitudinal axis of the tube.
Metallic measuring tubes, which, of course, must not be ferromagnetic so as not to interfere with the magnetic field, are commonly provided with an electrically nonconducting layer on the inner surface which serves as insulation; in the case of plastic or ceramic measuring tubes, such a layer is not necessary.
The induced voltage is picked off by means of galvanic or capacitive electrodes. Galvanic electrodes extend through the wall of the measuring tube, so that they contact the fluid, while capacitive electrodes are so mounted in the wall of the measuring tube as not to contact the fluid, or are mounted on the wall of the measuring tube.
U.S. Pat. No. 5,403,533 discloses a process for manufacturing a metallic measuring tube of an electromagnetic flow sensor which has an insulation on its inner surface, but no metal flanges, and wherein
at each end of the measuring tube, a tube thickening provided with an external undercut is formed integrally on the measuring tube, PA1 a tubular insulating lining which is longer than the measuring tube is drawn into the lumen of the measuring tube, and PA1 the projecting ends of the measuring tube are drawn over the tube thickening into intimate contact with the respective undercut, where they are secured with an annular mounting if necessary. PA1 at each end, the respective metal flange is formed on integrally, PA1 the front side of the metal flange being provided with projections and/or depressions, PA1 the depressions being filled with the insulation, PA1 the projections being embedded in the insulation. PA1 slipping the respective metal flange at each end of the metal tube over the outside of the latter in such a way PA1 that a front side of the metal flange projects beyond the respective end of the metal tube by an amount at least equal to the thickness of the insulation; PA1 permanently and tightly joining a rear side of the respective metal flange to the outside of the metal tube; PA1 prior to slipping the metal flanges over the metal tube, providing each of the metal flanges, from the respective front side, with a flute PA1 which forms a first annular groove at its inner end; PA1 introducing into the lumen of the metal tube a mandrel PA1 whose diameter is determined by the desired thickness of the insulation, and PA1 which has a filler inlet and two end flanges, one at each end, PA1 which cover the flutes of the respective flanges at the front side; PA1 filling in the insulation through the filler inlet in a liquid state; and PA1 allowing the insulation to solidify and removing the mandrel.
JP-B 5-48846, which is based on JP-A 61-294316, discloses a metallic measuring tube of an electromagnetic flow sensor which has an inner insulation and metal flanges and in which
DE-A 31 01 562 shows a similar design, but additionally a recess on that inner side of each flange which is the continuation of the inside surface of the metallic measuring tube.
British Patent 1,111,981 discloses an electromagnetic flowmeter for highly conductive liquids with a metallic measuring tube provided with a lining of steel which has belled ends which rest on the end faces of the respective flanges.